The present invention relates to an improved displacement detector for an encoder which is well suited for detection of linear or angular displacement.
One typical circuit for converting analog angular signals into corresponding digital signals includes an angle detector which is made up of a synchro control and a Scott-connected transformer. When a carrier wave signal sin.omega..sub.o t is supplied to the rotor of the synchro control transformer, its stator generates a synchronizing signal corresponding to the rotation angle .theta. of the rotor. When this synchronizing signal is supplied to the primary side of the Scott-connected transformer, the secondary side of the transformer generates analog signals sin.theta..sin.omega..sub.o t and cos.theta..sin.omega..sub.o t which are passed to respective demodulators. The demodulators remove the carrier wave components, i.e. sin.omega..sub.o t, from the analog signals to issue angle signals sin.theta. and cos.theta. which are applied to respective analog multipliers. Output signals from the mutlipliers are applied to a common reducer which issues a difference signal sin(.theta.-.PHI.). The difference signal sin(.theta.-.PHI.) is passed to an up-down counter via a voltage control oscillator. The count value .PHI. of this counter is applied to a function generator which then, depending on the count value .PHI. of the counter, issues sine and consine signals (sin .PHI. and cos .PHI.). These signals are passed to the above-described analog multipliers for multiplication purposes.
With this system, the analog multipliers issue output signals sin.theta..cos.PHI. and cos.theta..sin.PHI. and the output signal of the reducer is given by the following equation: EQU sin.theta..cos.PHI.-cos.theta..sin.PHI.=sin(.theta.-.PHI.)
according to the trigometric identiy sin(A.+-.B)=sinA.cosB.+-.cosA.sinB.
As a result, the output signal from the voltage control oscillator has a frequency corresponding to the value of sin(.theta.-.PHI.). Since the output signal .PHI. of the up-down counter changes depending on the value of sin(.theta.-.PHI.), the system operates as a sort of phase-locked loop which makes the value of sin(.theta.-.PHI.) be zero, i.e. makes .theta. equal to .PHI.. When .PHI. includes about 10 bits (i.e. when the output of the counter is about 10 bits), the system has a degree of resolution sufficient to divide the angular range of 0 to 2.pi. into 1024 sections.
With this conventional displacement detector, most signals are processed in analog form and, as a consequence, the use of many analog circuit elements limits the ability to form an integrated system. Presently, even a hybrid integrated circuit has a dimension of about 50.times.50 mm. In addition, variations in temperature of the voltage control oscillator and/or power source voltage tend to cause drift, thereby lowering reliability of the detection.